Influence of climate change on the frequency of daytime temperature inversions and stagnation events in the Po Valley: historical trend and future projections Stefano Caserini a, ⁎, Paolo Giani a , Carlo Cacciamani b , Senem Ozgen a , Giovanni Lonati a a Department of Civil and Environmental Engineering, Politecnico Milano, P.za L. da Vinci 32, Milano, 20133, Italy b ARPA Emilia-Romagna, Servizio Idro-Meteo-Clima, via Silvani 6, Bologna, 40122, Italy abstract article info Article history: Received 23 June 2016 Received in revised form 13 September 2016 Accepted 30 September 2016 Available online 03 October 2016 This work analyzes the frequency of days characterized by daytime temperature inversion and air stagnation events in the Po valley area. The analysis is focused on both historical series and future projections under climate change. Historical sounding data from two different Italian stations are used as well as future projections data, provided by CMCC-CCLM 4-8-19 regional climate model (MED-CORDEX initiative). A new method to detect layers of temperature inversion is also presented. The developed method computes the occurrence of a temper- ature inversion layer for a given day at 12 UTC without a detailed knowledge of temperature vertical profile. This method was validated using sounding data and applied to the model projections, under two different emissions scenarios (RCP4.5 and RCP8.5). Under RCP4.5 intermediate emissions scenario, the occurrence of temperature in- versions is projected to increase by 12 days/year (around + 10%) in the last decade of 21st century compared to 1986–2005 average. However, the increase in temperature inversions seems to be especially concentrated in the warm period. Under RCP8.5 extreme scenario, temperature inversions are still projected to increase, though to a lesser extent compared to RCP4.5 scenario (+6 days/year in the last decade of 21st century). A similar trend was found also for air stagnation events, which take into account the variation of precipitation pattern and wind strength. The expected increases are equal to +13 days/year and +11 days/year in the last decade of 21st cen- tury compared to 1986–2005 average, under RCP4.5 and RCP8.5 scenarios respectively. © 2016 Elsevier B.V. All rights reserved. Keywords: Med-CORDEX Atmospheric lapse rate Lapse rate inversion Air quality Atmospheric stability Air stagnation 1. Introduction The Po Valley area is characterized by one of the highest population density in Europe as well as by one of the poorest air quality (EEA, 2014). Since the average emissions per capita are similar to other Euro- pean urban areas (EEA, 2014), poor air quality is chiefly related to the adverse local climate (Carbone et al., 2010; Ferrero et al., 2011). As a matter of fact, the Po Valley basin is characterized by one of the lowest wind speed in Europe, on average between 2 and 2.5 m s -1 . During win- ter, the wind speed is even lower, around 1.5 m s -1 on average (Arpa Emilia-Romagna, 2013). Moreover, temperature inversions are very fre- quent, especially during the cold period when the height of Planetary Boundary Layer (PBL) rarely exceeds 450 m (Bigi et al., 2012). Temper- ature inversions reduce vertical dispersion ventilation into the free tro- posphere. Hence, primary pollutants tend to accumulate and secondary pollutants tend to form in a shallow layer near the surface (Perrino et al., 2014; Sandrini et al., 2014). However, as climate is changing, a growing interest on the relation- ship between global warming and the diffusive properties of atmo- sphere has occurred over the last few years. According to Kirtman et al. (2013), the frequency of stagnation events will decrease on a global scale, although some increases are still possible in some regions. Accord- ing to Horton et al. (2012), stagnation events are expected to increase by 12–25% by the end of 21st century in the Mediterranean area. Fur- thermore, climate change-induced variations of temperature and hu- midity can influence atmospheric chemical reactions and thus the formation of secondary pollutants (Stocker et al., 2013). As regards Italy, some studies (Pasini and Cipolletti, 2007; Giulianelli et al., 2014) showed that global warming has already influenced atmo- spheric diffusive properties, although a clear trend cannot be detected due to the limited length of the assessment period. However, a 50% de- crease of fog events, typically related to temperature inversions, has been observed in the Po Valley from the early 1990s (Giulianelli et al., 2014). This large decrease has been also found in other regions of the world, such as in California (Johnstone and Dawson, 2010). Potential causes of this trend might be the increasing temperature and the decline of available condensation nuclei due to the recent implementation of air quality policies focused on particulate matter emissions. However, since Atmospheric Research 184 (2017) 15–23 ⁎ Corresponding author. Tel.: +39023996414; fax: +390223996430. E-mail address: stefano.caserini@polimi.it (S. Caserini). http://dx.doi.org/10.1016/j.atmosres.2016.09.018 0169-8095/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Atmospheric Research journal homepage: www.elsevier.com/locate/atmosres